Metamaterials are engineered materials or complex composites that gain their properties not from the chemical composition of their components, but instead from the design of their microstructure. Metamaterials can be designed to display unusual material behavior when subject to electromagnetic, acoustic, or elastic waves. They may exhibit properties such as negative refractive index, negative bulk modulus, or negative mass; properties which are desirable for many engineering purposes but not typically observable in materials found in nature. The unique properties of metamaterials are often gained through the use of inclusions and structural features.
Sonic or phononic metamaterials are an example of such a material, and have been developed for the manipulation of acoustic waves. When these periodic composite materials interact with acoustic waves, they exhibit wave filtering behavior, creating band gaps or stop bands within the frequency spectrum. Within the band gap frequency range, the applied acoustic waves cannot propagate and the incident energy is dispersed or scattered from the medium.